Sanitaryware is glazed, vitrified whiteware ceramic having a sanitary service function as bathtubs, sink basins, toilets, etc. Many attempts have been made by the ceramic industry to improve the quality and strength of ceramic products, particularly lowering the cracking and breakage rate of sanitaryware. Such attempts have included incorporating fibrous materials into the sanitaryware. Standard sanitaryware bodies show a typical brittle fracture pattern in which minor energy is required to propagate a crack once the crack has been initiated. Previous studies have shown that adding glass fibers to sanitaryware ceramic mixtures modify fracture behavior, increase the modulus of rupture and work of fracture values, and inhibit the propagation of cracks. However, the use of special glass fibers may not be suitable for use in sanitaryware manufacturing due to the cost, health risks, and the physical properties of the glass fibers. In addition, the glass may not exhibit plastic-like behavior during firing.
Sanitaryware bodies containing fibers exhibit an inhibited fracture pattern. In these fiber containing bodies, a significant amount of energy is required to propagate cracks due to the fibers binding the body together. Chopped fiberglass, has been investigated as a possible additive material to enhance reinforcement. However, chopped fiberglass and other fiber-like materials including cellulose fiber, and processed mineral fibers (rock wool), silicon fiber, carbon fiber, and potassium titanate have cost or processing limitations, inhibiting their commercial usefulness. For example, chopped fiberglass and rock wool are prohibitively expensive, while cellulose fiber, and processed mineral fibers must first be processed. Others, such as potassium titanate, pose potential health risks.
Wollastonite is a natural calcium silicate that has a theoretical composition of CaSiO.sub.3 (which may also be written CaO.SiO.sub.2). The chemical composition of wollastonite is understood to be about 48.3% calcia (CaO) and about 51.7% silica (SiO.sub.2). A significant use of wollastonite is in ceramics such as wall tile, where it promotes low shrinkage, good strength, low warparge, and fast firing. It is also used in porcelain, as a filler in paint, plastics and papers, as an electrode coating, as a protective slag for continuous casting carbon steel and casting silicon steel sheet, and in asbestos replacement. It is believed that one of the major reasons surrounding the successful use of wollastonite in ceramic products is that the silica is not present as a separate phase in the ceramic product. Silica has a significantly different thermal coefficient of expansion than the other phases present in the fired ceramic product. It also undergoes a phase transformation during the cooling of the ceramic product. Both of these factors cause cracking of the ceramic product unless a slow cooling rate is used. By eliminating the free silica phase, higher firing and cooling rates can be employed.
The use of wollastonite for reinforcement of sanitaryware has not been fully investigated. Sanitaryware is typically defined as glazed, vitrified whiteware fixtures having a sanitary service function, such as for lavatory basins, bathtubs, and toilets, etc. Sanitaryware differs from wall tile in many respects, such as size and shape. Generally, wall tile is limited to a small size, such as a 3".times.3" tile. Further, tile is generally flat. However, sanitaryware is generally large in size(i.e. toilet, tub), and multiplanar. Therefore, the stresses and expansion dynamics of sanitaryware versus tile are considerably different. The firing conditions also vary with the use of plaster molds for sanitaryware and are significantly different than for other types of ceramics. Further, the use of wollastonite in tile is generally for its chemical properties of bound silica.
The present invention is directed to the discovery of the use of wollastonite, in the form of needle-like fibers or shards, as an additive to improve ceramic sanitaryware performance by inhibiting crack or microcrack propagation. Since wollastonite is relatively inexpensive, naturally occurring, non-carcinogenic and is easy to handle and transport, it would be highly beneficial to the ceramic industry, and specifically to the sanitaryware industry, if wollastonite could be incorporated into ceramic and sanitaryware products.